1. Field of the Invention
The present invention relates to a technology for compressing and decompressing data with a reduction of data size and an increased security.
2. Description of the Related Art
Recently, with the spread of the Internet and mobile terminals, there are increasing concerns about data stored in respective devices, reduction of data size communicated between respective devices, and security (data secrecy). As the technique relating to the reduction of data size, various compression techniques have been disclosed, and by using such compression techniques, needs for reduction of data size can be satisfied.
As the technique relating to data secrecy, a method of adding a password function to the data and a method of encrypting data have been disclosed. By using these techniques and the compression technique together, both reduction of the data size and the data secrecy can be realized.
For example, there is a data compression method with a password function in which password information is attached to the compressed data, as the technique for realizing both reduction of the data size and the data secrecy. According to the data compression method with the password function, compressed data including the password information is created by adding a password to the compressed data. Only when a password newly input on the decompression side matches the password input on the compression side, decompression of the compressed data can be performed.
Japanese Patent Application Laid-open No. 2001-119588 discloses a technique relating to a fax machine in which by inserting a dummy data in the compressed image data, the secrecy of the image data is maintained, thereby realizing both reduction of the data size and data secrecy.
However, in the data compression method with the password function, the compressed data itself is not subjected to a process for concealing the data. Therefore, there is a problem in that the compressed data can be decompressed only by removing the password information from the compressed data including the password information. That is, in such a data compression method, the security level of the compressed data is low.
Also in the technique disclosed in Japanese Patent Application Laid-open No. 2001-119588, the compressed data itself (image data after compression) is not subjected to the process for concealing the data, and hence, there is the same problem such that only by removing the inserted dummy data, the compressed data can be decompresseded.
Therefore, it is very important how to realize a data compression method and an decompression method that can realize both reduction of data size and the data secrecy by giving the secrecy to the compressed data itself.